The present invention relates to an electric double layer capacitor, particularly to an electric double layer capacitor having a low resistance, a high withstand voltage and excellent reliability.
As a shape of a conventional electric double layer capacitor, there may be a coin type wherein an element having a separator sandwiched between a pair of polarized electrodes composed mainly of activated carbon formed on current collectors, is accommodated together with an electrolytic solution in a metal casing, which is then sealed by a metal cover via a gasket, or a cylindrical type wherein an element having a pair of polarized sheet electrodes wound with a separator interposed therebetween, is accommodated together with an electrolytic solution in a metal casing, which is then sealed so that the electrolytic solution will not evaporate from an opening of the casing.
Further, as one for a large current and large capacitance, a lamination type electric double layer capacitor has also been proposed wherein an element having many polarized sheet electrodes laminated via a separator disposed therebetween, is incorporated (JP-A-4-154106, JP-A-3-203311, JP-A-4-286108). Namely, rectangular polarized sheet electrodes are used as a positive electrode and a negative electrode, and they are alternately laminated with a separator interposed therebetween, to form an element, which is then accommodated in a casing in such a state that a positive electrode lead member and a negative electrode lead member are connected by caulking to the terminals of the positive and negative electrodes, respectively, then the element is impregnated with an electrolytic solution, and the casing is closed with a cover.
As an electrolytic solution for a conventional electric double layer capacitor, not only an aqueous type electrolytic solution containing a mineral acid such as sulfuric acid, an alkali metal salt or an alkali, but also various non-aqueous electrolytic solutions have been used. As the solvent for such non-aqueous electrolytic solutions, propylene carbonate, xcex3-butyrolactone, acetonitrile, dimethyl formamide (JP-A-49-068254) or a sulfolane derivative (JP-A-62-237715), has been known. When the withstand voltages are compared, the aqueous type electrolytic solution has a withstand voltage of 0.8 V, while the non-aqueous electrolytic solution has a withstand voltage of from 2.5 to 3.3 V. The electrostatic energy of a capacitor corresponds to the square of the withstand voltage. Accordingly, from the viewpoint of the electrostatic energy, the non-aqueous type electrolyte is more advantageous. However, such a solvent having a high dielectric constant, in which an electrolyte is highly soluble, usually has a high viscosity and has a problem such that high electrical conductivity can not be obtained although an electrolytic solution having a high electrolyte concentration can be obtained. On the other hand, a chain ether, a chain ester or a chain carbonate, which is a low viscosity solvent, has a low dielectric constant and thus has a problem that it is substantially incapable of dissolving an electrolyte, whereby high electrical conductivity can not be obtained.
Propylene carbonate which is commonly used as a solvent for an electrolytic solution for an electric double layer capacitor, has a high dielectric constant and is a solvent, of which the viscosity is not relatively high. Thus, it is preferably employed. However, it has had a problem that propylene carbonate is likely to decompose when a voltage of about 2.9 V or higher is applied. Whereas, sulfolane which is electrochemically stable, has a high withstand voltage and will not be decomposed until a voltage as high as 3.4 V is applied, but it has had problems such that the solubility of an electrolyte is low, and the melting point is high.
Under these circumstances, it is an object of the present invention to study particularly an electrolytic solution to solve the above problems of the prior art and to provide an electric double layer capacitor having a high withstand voltage, a low resistance and excellent reliability.
The present invention provides an electric double layer capacitor having a pair of polarized electrodes and an electrolytic solution capable of forming an electric double layer at the interface with the polarized electrodes, wherein the electrolytic solution comprises a salt of the Formula 1 as the electrolyte and at least dimethyl carbonate as a solvent:
R1R2R3R4N+Xxe2x88x92xe2x80x83xe2x80x83Formula 1
wherein R1 is a n-propyl group and each of R2, R3 and R4 which are independent of one another, is a methyl group or an ethyl group, provided that two selected from R1 to R4 may together form a tetramethylene group, and Xxe2x88x92 is an anion.
Heretofore, as the cation of an electrolyte for an electrolytic solution for an electric double layer capacitor, (C2H5)4N+, (C2H5)4P+ or the like, which has high electrical conductivity, has been used. Also, (C2H5)3(CH3)N+, which has a high solubility in a solvent, has been proposed, but no adequate solubility to a solvent having a low dielectric constant and low viscosity, has been obtained. Among solvents having a low dielectric constant and low viscosity, a chain carbonate is electrochemically stable, but dimethyl carbonate, ethylmethyl carbonate or the like has had a problem that it can not substantially dissolve an electrolyte by itself as a single solvent.
The electrolyte to be used for the electrolytic solution for the electric double layer capacitor of the present invention is a chain quaternary ammonium cation represented by R1R2R3R4N+, wherein R1 is a n-propyl group, and each of R2, R3 and R4 which are independent of one another, is a methyl group or an ethyl group. Here, two selected from R1 to R4 may together form a tetramethylene group to provide a cyclic pyrrolidinium cation (each of the remaining two groups is the monovalent alkyl group as defined above).
Such a cation is electrochemically stable, and the electrolyte having such a cation has high solubility to dimethyl carbonate as a solvent having a low dielectric constant and a low viscosity. Accordingly, when a solution having such an electrolyte dissolved in dimethyl carbonate or in a solvent mixture containing dimethyl carbonate, is used as an electrolytic solution, it is possible to provide an electric double layer capacitor which is excellent in durability even when used for a long period of time and which thus has high reliability.
Among cations represented by R1R2R3R4N+ in the Formula 1, a cation represented by any one of the following Formulae 2 to 6 may be mentioned as a specific preferred example:
(C3H7)(C2H5)3N+xe2x80x83xe2x80x83Formula 2
(C3H7)(C2H5)2(CH3)N+xe2x80x83xe2x80x83Formula 3
(C3H7)(C2H5)(CH3)2N+xe2x80x83xe2x80x83Formula 4
Among them, particularly preferred is a cation represented by any one of the Formulae 3 to 6, which contains three types of groups among a tetramethylene group, a n-propyl group, an ethyl group and a methyl group. The salt having a cation containing such three types of groups, has a high solubility to a solvent and can thus be highly concentrated, as compared with a salt having a cation containing two types of groups, and high electrical conductivity can be realized by making the electrolytic solution to be highly concentrated.
Further, if the carbon chain of the group bonded to the nitrogen atom is long, the solubility to a solvent increases, and the concentration can be made high, such being preferred. On the other hand, if the carbon chain of the group bonded to the nitrogen atom is long, the ion radius becomes large, and such ions tend to hardly enter into fine pores of the electrodes, whereby the amount of ions forming an electric double layer per unit volume, will be small, and from such a viewpoint, the shorter the carbon chain, the better. Specifically, particularly preferred is a salt having a cation represented by any one of the Formulae 2 to 6, wherein the average of the carbon chains of from R1 to R4 is about 2, in view of the solubility to a solvent, the amount of ions and the electrical conductivity of the solution.
The anion of the electrolyte contained in the electrolytic solution for the electric double layer capacitor of the present invention is preferably an anion selected from the group consisting of BF4xe2x88x92, PF6xe2x88x92, CF3SO3xe2x88x92 and (CF3SO2)2Nxe2x88x92. Particularly preferred is BF4xe2x88x92 from the viewpoint of e.g. the solubility to a solvent, the electric conductivity of the solution and the electrochemical stability.
In the present invention, the solvent for the electrolytic solution may be composed solely of dimethyl carbonate, but is preferably a solvent mixture of dimethyl carbonate with other solvents. As such other solvents, known solvents may be used. For example, a cyclic carbonate such as propylene carbonate, ethylene carbonate or butylene carbonate, a chain carbonate such as ethylmethyl carbonate or diethyl carbonate, a cyclic lactone such as xcex3-butyrolactone or xcex3-valerolactone, a nitrile such as acetonitrile or glutaronitrile, a sulfolane derivative such as 3-methylsulfolane, dimethylformamide, 1,2-dimethoxyethane, nitromethane and trimethylphosphate, may, for example, be mentioned. It is preferred to use one or more of them.
Further, in the present invention, the solubility of the electrolyte to dimethyl carbonate is high, whereby a solvent having an extremely low polarity, which could not be used heretofore; may be incorporated to the electrolytic solution.
In the electric double layer capacitor of the present invention, it is particularly preferred to use, as the solvent for the electrolytic solution, a solvent mixture of dimethyl carbonate with a solvent having a high withstand voltage and a large dielectric constant. Specifically, a solvent mixture comprising dimethyl carbonate, and sulfolane or butylene carbonate, is preferred.
The electrical conductivity of the electrolytic solution having dimethyl carbonate combined as a solvent with the electrolyte in the present invention, reaches the maximum when the concentration of the electrolyte is brought to a level of 2.2 mol/kg (in a case where the electrolyte represented by the following Formula 7, is employed) or to a level of 2.6 mol/kg (in a case where the electrolyte represented by the following Formula 8 is employed), and even if the concentration is further increased, the electrical conductivity will no longer increase. Further, if the concentration of the electrolyte is too high, the ammonium salt as the electrolyte is likely to precipitate in a cold atmosphere, whereby the stability is likely to deteriorate. On the other hand, if the concentration is too low, the internal resistance increases. For the above reasons, the concentration of the electrolyte is preferably from 1.4 to 2.5 mol/kg in a case where the electrolyte represented by the following Formula 7 is employed, or from 1.8 to 3.0 mol/kg in a case where the electrolyte represented by the following Formula 8 is employed, particularly preferably, the concentration of the electrolyte is made to be from 1.6 to 2.2 mol/kg or from 2.0 to 2.6 mol/kg, respectively. By the mass ratio, the proportion of the electrolyte is preferably from 30 to 60% in the total mass of the electrolytic solution.
(C3H7)(C2H5)2(CH3)N+BF4xe2x88x92xe2x80x83xe2x80x83Formula 7
On the other hand, dimethyl carbonate is contained preferably in an amount of from 20 to 70% in the total mass of the electrolytic solution. When the solvent of the electrolytic solution is composed solely of dimethyl carbonate, the proportion of dimethyl carbonate is preferably from 40 to 70%. However, in the case of a solvent mixture of dimethyl carbonate with other solvents, it is preferred to adjust the proportion of dimethyl carbonate to be at least 20%, while maintaining the proportion of the electrolyte to be from 30 to 60%. If dimethyl carbonate is less than 20%, the solubility of the electrolyte tends to be low, and the concentration of the electrolyte may not be sufficiently increased.
In a case where a solvent mixture of dimethyl carbonate with sulfolane is used as the solvent for the electrolytic solution, the electrical conductivity will be higher than the case where dimethyl carbonate is used alone as the solvent. In such a case, sulfolane is contained preferably in an amount of at most 30%, particularly preferably at most 25%, in the total mass of the electrolytic solution. If the content of sulfolane exceeds 30%, it tends to be difficult to increase the concentration of the electrolyte. Further, it is preferred to have sulfolane contained in an amount of at least 5%, particularly preferably at least 15%, by mass ratio, so that the effect for increasing the electrical conductivity can readily be obtained.
Further, in a case where a solvent mixture of dimethyl carbonate with butylene carbonate is used as the solvent for the electrolytic solution, the electrical conductivity will be higher than the case where dimethyl carbonate is used alone as the solvent, such being preferred. In such a case, it is preferred to have butylene carbonate contained in an amount of at most 50%, particularly preferably at most 40%, in the total mass of the electrolytic solution. If the content of butylene carbonate exceeds 50%, it tends to be difficult to increase the concentration of the electrolyte. Further, it is preferred to have butylene carbonate contained in an amount of at least 20%, particularly preferably at least 30%, by mass ratio, so that the effect for increasing the electrical conductivity can readily be obtained.
The electrolytic solution in the present invention is a non-aqueous electrolytic solution, and the smaller the metal impurities and water content, the better. Usually, one having a water content of at most 10 ppm, is suitably employed.
The polarized electrodes to be used for the electric double layer capacitor of the present invention may be ones made mainly of an electrochemically inactive material having a high specific surface area, specifically those made mainly of activated carbon, fine metal particles or fine electrically conductive oxide particles. Among them, it is preferred to use those having an electrode layer comprising a powder of carbon material having a high specific surface area such as activated carbon, formed on the surface of metal current collectors.
Specifically, the electrode layer is formed preferably by using, as the main component, a powder of carbon material such as activated carbon or polyacene having a large specific surface area (specific surface area: about 200 to 3,000 m2/g), adding thereto carbon black, acetylene black, Ketjenblack or carbon whisker as a conductive material, and polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF) or carboxymethylcellulose as a binder, kneading the mixture in the presence of a liquid lubricant such as an alcohol, molding the mixture into a sheet by rolling, followed by drying to obtain a sheet-form molded product, which is bonded by heat pressing or bonded by means of an electrically conductive adhesive or the like to both sides of a metal current collector.
Further, instead of kneading, a solvent capable of dissolving the above binder or a solvent mixture containing such a solvent (water, N-methylpyrrolidone, etc.) may be mixed with activated carbon, a conductive agent and a binder to obtain a slurry, which may be coated on both sides of a metal current collector and dried to form the electrode layer. The thickness of such an electrode layer is not particularly limited, but is usually from about 10 xcexcm to 0.5 mm.
As the activated carbon material, one derived from a natural plant tissue such as coconut shell, a synthetic resin such as a phenolic resin or a fossil fuel such s coal, coke or pitch, may be employed. As an activating method for activated carbon, steam activation or alkali activation (particularly activation by KOH) may be applied, although it may vary depending upon the raw material to be employed. Activated carbon derived from a natural plant tissue or a fossil fuel, contains a relatively large amount of metal impurities, and accordingly, washing with e.g. an acid is usually required. Similarly, activated carbon obtained by alkali activation contains a large amount of an alkali metal used for the activation or metal impurities brought from an activation apparatus due to the reaction with the alkali, and accordingly a washing operation will be required. Among them, steam activated carbon made of a synthetic resin as a raw material, is most preferred from the viewpoint of metal impurities.
The element construction of the electric double layer capacitor of the present invention is not particularly limited, and the present invention can be applied to any one of a coin type structure, a cylindrical structure or an angular structure. For example, the coin type structure may be formed in such a manner that an element is formed by disposing a separator between a pair of electrodes having electrode layers composed mainly of activated carbon provided on current collectors, and the element is, together with an electrolytic solution, sealed in a coin type metal casing by a metal cover and a gasket which insulates both.
Whereas, the cylindrical structure may be formed, for example, in such a manner that a pair of strip-shaped electrodes, specifically a strip-shaped positive electrode having an electrode layer composed mainly of e.g. activated carbon formed on both sides of a metal current collector and a strip-shaped negative electrode having an electrode layer of the same construction formed on both sides of a metal current collector, are alternately laminated via a strip-shaped separator and wound to obtain a wound element, which is then accommodated in a cylindrical metal casing and impregnated with the electrolytic solution, whereupon the current collecting leads taken out from the positive electrode and the negative electrode, respectively, are connected, respectively, to the electrode terminals provided, for example, on an electrically insulating sealing cover, and the sealing cover is fit to the metal casing.
The angular structure may be formed in such a manner that electrode layers are formed on both sides of a rectangular metal current collector, a plurality of positive electrodes and a plurality of negative electrodes, each provided with a current collecting lead, are alternately laminated via a separator, to form a laminated element having current collecting leads taken out, which is accommodated in an angular metal casing and impregnated with the electrolytic solution, whereupon a sealing cover is fit on the angular casing.
The current collector may be made of any metal so long as it is electrochemically or chemically corrosion resistant. In the case of a coin type structure, the housing member such as the metal sealing cover or the metal casing, may serve as a current collector, in many cases. As the current collector in the case of the cylindrical structure or the angular structure, it is preferred to employ a surface-roughened foil or net made of a metal such as aluminum, stainless steel, nickel or tantalum, particularly a foil or net made of a stainless steel, aluminum or an alloy containing it. More preferred is an aluminum foil having a purity of 99.9%, particularly preferably 99.99%. In the present invention, it is preferred to employ a metal current collector made of such a metal foil and having a thickness of from 10 xcexcm to 0.5 mm.
In the case of a cylindrical structure or an angular structure, current collecting leads will be provided to the metal current collectors. It is preferred to provide a tape- or ribbon-shaped portion on a current collector having no electrode layer formed thereon and to bond an electrically conductive tab terminal, wire, tape, ribbon or the like by e.g. welding to such a portion to form a current collecting lead. Otherwise, a portion having no electrode layer formed, is provided at a part of a current collector, so that such a portion may be used as a current collecting lead. Specifically, for example, in the case of a cylindrical structure, a strip portion having no electrode layer formed, may be provided along one end in the longitudinal direction of a strip current collector, and the counter electrode is overlaid via a separator so that the strip portion is located at the opposite end, and the assembly is wound to obtain an element, whereby both end surfaces (the above strip portions) of the element can be used as current collecting leads.
The separator of the present invention is not particularly limited, so long as it is a porous separator so that ions can permeate therethrough. A fine porous polyethylene film, a fine porous polypropylene film, a polyethylene non-woven fabric, a polypropylene non-woven fabric, a polypropylene non-woven fabric, a glass fiber incorporated non-woven fabric, a glass mat, cellulose paper, craft pulp, sisal hemp or Manila hemp, may, for example, be preferably employed. The thickness of the separator is preferably from 20 to 200 xcexcm, particularly preferably from 30 to 100 xcexcm. From the viewpoint of the absorptivity for the electrolytic solution, the liquid maintaining property and the internal resistance, the higher the porosity, the better. However, as the porosity is high, defects such as pinholes are likely to increase, thus leading to self discharge failure. Accordingly, the porosity is usually preferably within a range of from 50 to 90%, more preferably within a range of from 60 to 85%.
Now, the present invention will be described in further detail with reference to Examples and Comparative Examples. However, it should be understood that the present invention is by no means restricted by such specific Examples.